InnoSoilPhos

Minable phosphate rock, the basis for the production of phosphate fertilisers, is a limited georesource that is also mainly found in politically unstable regions of the world. The EU has categorised phosphate as a critical raw material. On the other hand, phosphate inputs into water bodies have a negative impact on their quality; the phenomenon of eutrophication of water bodies is largely caused by excessive phosphate concentrations.

Sustainable phosphate management requires solutions to be developed to ensure that crops are adequately supplied and water bodies are protected. To this end, a multi- and interdisciplinary approach was used to develop an understanding of the process of P conversion in the soil-plant-water system in order to identify a large number of ‘levers’ that can be used for the sustainable management of this essential main nutrient. The methods used included quantum chemical modelling, molecular and microbiological methods, soil chemistry studies and experiments, metadata analyses, conventional cultivation trials on a container, plot and field scale, monitoring of discharge behaviour at catchment level and, finally, governance research methods.

Quantum chemical modelling at the atomic level led to the realisation of how phosphate binding to iron minerals depends on pH, the structure of the phosphate compounds and the coverage of the minerals with organic molecules. Practical instructions for action include controlling the pH value by liming and the humus content by increasing the supply of organic residues in order to minimise P fixation. At the molecular level, new insights were gained into the genes that control P utilisation in soil microorganisms and plants. Of the relevant groups of bacteria, those that are naturally associated in the soil with intercropping plants suitable for cultivation (e.g. Bradyrhizobiacae on Serradella) or that are suitable as authorised strains for the development of inoculants for P recycling products (e.g. Bacillus velezensis) are to be promoted. Such P-recycling products, e.g. biochars from P-rich materials such as bones and sewage sludge, have been tested in a variety of experimental approaches. The results consistently showed that, for example, a bone char loaded with sulphur had a similarly good fertilising effect as the commercial fertiliser tripel superphosphate. Several years of plot and field trials with bone char and biochar from sewage sludge led to the same result. The in-depth evaluation of P fertilisation trials in Germany and the EU led to the conclusion that the importance of P fertilisation for yield levels was probably significantly overestimated in the past.

The recommended P fertilisation did not always lead to higher yields in all soil P content classes. Even after suspending P fertilisation for 36 years, significant reductions in yield were only observed for sugar beet, but not for winter cereals. It follows that a more economical use of P fertilisers makes economic and ecological sense. Investigations of P discharge in drained lowland catchment areas led to the realisation that discharges with the base flow are negligible.

In contrast, heavy rainfall events lead to peaks in runoff and P discharges. This can be explained by erosion, superficial and ‘internal’ discharges in the soil profile. Avoiding such runoff peaks is extremely challenging and has not yet been satisfactorily solved technically. Therefore, the limitation of agricultural P inputs remains The same result was achieved in several years of plot and field trials with bone char and biochar from sewage sludge. The in-depth evaluation of P fertilisation trials in Germany and the EU led to the conclusion that the importance of P fertilisation for yield levels was probably significantly overestimated in the past.

The InnoSoilPhos project has shown a variety of ways in which fertiliser P can be saved and replaced by recycled products. Politically, better P-cycle management, more efficient P use and long-term food security require a mix of instruments, and the greatest pressure to achieve this could be exerted by controlling the volume of livestock farming at EU level. A restructuring of EU agricultural subsidies according to the principle of ‘public money only for public services’ and stronger regulatory requirements for P recycling would also help.

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Deinen Boden besser verstehen - Prof. Dr. Christel Baum, Universität Rostock

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